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From Wikipedia, the free encyclopedia

Back injuries result from damage, wear, or
trauma to the bones, muscles, or other tissues of the back. Common
back injuries include sprains and strains, herniated disks, and
fractured vertebrae.[1] The lumbar is often the site of back
pain. The area is susceptible because of its flexibility and the
amount of body weight it regularly bears.[2] It is
estimated that low-back pain may affect as much as 50 to 70 percent
of the general population in the United States.[3]

Low-back pain is often the result of incorrect lifting methods
and posture. Repetitive lifting, bending, and twisting motions of
the torso affect both the degree of severity and frequency of
low-back pain. In addition, low-back pain may also be the result of
bad lifting habits. Sedentary lifestyles most often lead to weak abdominal muscles and hamstrings. This causes
the stronger muscles which have remained strong to pull the body
away from its optimal anatomical form. The imbalanced muscles cause
people to continue to perform these repetitive actions. This
results in misplaced force application within the spine, often
resulting in hemorrhage of disks within the spinal column.

Back
injuries and lifting

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Low-back biomechanics of
lifting

The lower back is the most vulnerable to injury due to its
distance from the load handled by the hands. Both the load and the
weight of the upper torso create significant stress on the body
structures at the low back, especially at the disc between the
fifth lumbar and the first sacral vertebrae (known as the L5/S1
lumbosacral disc). Figure 1 below shows the reactive forces and
moments on the sacral disc.

According to the second condition of static equilibrium, we
have

(moments at the L5/S1 disc) = 0 [Eq. 1]

This equation indicates that a clockwise rotational moment of
the torso must be counteracted by a counterclockwise rotational
moment, which is produced b the back muscles with a moment arm of
about 5 cm. Thus, when a person with an upper-body weight of
Wtorso lifts a load with a weight of Wload,
the load and upper torso create a combined clockwise rotational
moment that can be calculated as

Mload-to-torso = Wload* h + Wtorso*b [Eq. 1a]

where h is the horizontal distance from the load to the
L5/S1 disc, and b is the horizontal distance from the
center of mass of the torso to the L5/S1 disc. The counterclockwise
rotational moment produced by the back muscles is

Mback-muscle = Fback-muscle*5 (N-cm)

Then substituting Eq. 3 into Eq. 2, we find the following
equations.

Fmuscle*5 = Wload* h + Wtorso*b
Fmuscle = Wload* h/5 +
Wtorso*b/5

Since h and b are always much larger than 5 cm,
Fmuscle is always much greater than the sum of the
weight of the load and torso.

This equation indicates that for a lifting situation discussed
here, which is typical of many lifting tasks, the back muscle force
is eight times the load weight and four times the torso weight
combined. The above equation tells us that the back muscle force
would be 3,800 N, which may exceed the capacity of some people. If
the same person lifts a load of 450 N, the equation indicates that
the muscle force would reach 5,000 N, which is at the upper limit
of most people’s muscle capability. Farfan estimates that the
normal range of strength capability of the erector spinal muscle at
the low back is 2,200 to 5,500 N.[4]

In addition to the back muscle strength considerations, the
compression force on the L5/S1 disc must also be taken into
account. This can be estimated with the first condition of the
static equilibrium:

(forces at the L5/S1 disc) = 0 [Eq. 2]

Then

Fcompression = Wload*cos α +
Wtorso* cos α + Fmuscle

where α is the angle between the horizontal plane and the sacral
cutting plane, which is perpendicular to the disc compression
force.

If a person with a torso weight of 350 N lifts a load of 450 N,
from Eq. 6, the compression force on the L5/S1 disc is then:

A compression force of 5458 N on the L5/S1 disc can be hazardous
to many people.

In carrying out a lifting task, several factors influence the
load stress placed on the spine. In this biomechanics model, the
weight and the position of the load relative to the center of the
price are just two of the factors that are important in determining
the load on a spine. Other important factors include the degree of
twisting of the torso, the size and shape of the object, and the
distance the load is moved.

Calculating
injury-free lifting capabilities

One equation, known as the NIOSH lifting equation, provides a
method for determining two weight limits associated with two levels
of back injury risk. The first limit is called an action limit
(AL), which represents a weight limit above which a small portion
of the population may experience increased risk of injury if they
are not trained to perform the lifting task. The second limit,
called the maximum permissible limit (MPL) is calculated as three
times the action limit. This weight limit represents a lifting
condition at which most people would experience a high risk of back
injury.

The recommended weight limit (RWL) is the load value for a
specific lifting task that nearly all healthy workers could perform
for a substantial period of time without an increased risk of
developing lifting-related low-back pain and is calculated as
follows.[3]

HM – horizontal multiplier. Reflects the fact that disc
compression force increases as the horizontal distance between the
load and the spine increases. As a result, the maximum acceptable
weight limit should be decreased from LC as the horizontal distance
increases.

VM – vertical multiplier. The NIOSH lifting equation assumes
that the best originating height of the load is 30 inches (or
75 cm) above the floor. Lifting from near the floor (too low)
or high above the floor (too high) is more stressful that lifting
from 30 inches above the floor. DM – distance multiplier;
based on the suggestion that as the vertical distance of lifting
increases, physical stress increases

AM – asymmetric multiplier; torso twisting is more harmful to
the spine than symmetric lifting. Therefore, the allowable weight
of lift should be reduced when lifting tasks involve asymmetric
body twists. CM – coupling multiplier, whose value depends on
whether the load has good or bad coupling. If the loads have
appropriate handles or couplings to help grab and lift the loads,
it is regarded as good coupling. If the loads do not have
easy-to-grab handles or couplings, but are not hard to grab and
lift, it is fair coupling. Poor coupling is where the loads are
hard to grab and lift. FM – frequency multiplier, is used to
reflect the effects of lifting frequency on acceptable lift
weights. H - horizontal distance between the hands lifting the load
and the midpoint between the ankles.

V – vertical distance of the hands from the floor. D – vertical
travel distance between the origin and the destination of the lift.
A – angle of symmetry (measured in degrees), which is the angle of
torso twisting involved in lifting a load that is not directly in
front of the person. F – average frequency of lifting measured in
lifts/min

To quantify the degree to which a lifting task approaches or
exceeds the RWL, a lifting index (LI) was proposed. LI is the ratio
of the load lifted to the RWL, and is used to estimate the risk of
specific lifting tasks in developing low-back disorders and to
compare the lifting demands associate with different lifting tasks
for the purpose of evaluating and redesigning them.

Lifting tasks with:

LI > 1 – likely to pose an increased risk for some
workers

LI > 3 – many or most workers are at high risk of developing
low-back pain and injury.

Material
handling

Administrative
precautions

Strength testing of existing workers, which one study showed
can prevent up to one-third of work-related injuries by
discouraging the assignment of workers to jobs that exceed their
strength capabilities.

Training employees to utilize lifting techniques that place
minimum stress on the lower back.

Enhancing availability of material handling equipment such as
carts,
dollies or hand trucks.

Physical conditioning or stretching programs to reduce the risk
of muscle strain.

Design
parameters

Loads should be kept close to the body and located at about
tight or waist height if possible.

Large packages should not be presented to a worker at a height
lower than about mid-thigh, or about 30 in. above the floor
(Chaffin, 1997). An adjustable lift table can be used to assist
workers when handling large or heavy objects.

Minimize torso twisting in materials handling

Frequency of lifting should be minimized by adopting adequate
lifting and workrest schedules.

A reduction in the size or weight of the object lifted. The
parameters include maximum allowable weights for a given set of
task requirements; the compactness of a package; the presence of
handles, and the stability of the package being handled.

Adjusting the height of a pallet or shelf. Lifting which occurs
below knee height or above shoulder height is more strenuous than
lifting between these limits. Obstructions which prevent an
employee's body contact with the object being lifted also generally
increase the risk of injury.

Other factors such as whole body vibration, psychosocial
factors, age, sex, body size, health, physical fitness, and
nutrition conditions of a person, are also important in determining
the incidence rate and severity of low back-pain.

In a recent study it was determined that up to one-third of
compensated back injuries could be prevented through better job
design (ergonomics).

References

^"Back injuries".
MedlinePlus. U.S. National Library of Medicine and National
Institutes of Health. July 2, 2009. Accessed July 15, 2009.